Regulated DC Power supply with Op-amp

This is the last part of some lab work I am undertaking. Attached is a photo of the lab script. In the attachment is the circuit image and the questions.

The last two questions are trivial, naturally the difficulty I am having is in the brunt of (a).

Prerequisites
We havn't had a formal introduction to transistors yet, so we have been told to ignore that part and were given a simple description of what it is doing. We have been given a formula for regulation and for calculating the average value of a rectified waveform and another one for calculating voltage ripple.

Attempt
(a) is basically asking us to find the values of R1, R2, and R3.
The op-amp is operating as a comparator, taking a fraction of the output in as negative feedback.

To find R1, I have:

To find R1 and R2, I am using:

Sometimes I understand that it's ok to pic an arbitrary value for one resistor (and rearrange for the other) as long as it lies within a certain region. In this case I have no idea what that region is. Is it ok to pic an arbitary value for R2 and find R3, or should I consider something when deciding a value for R2?

It is meant to be a real circuit that we are going to build tomorrow. There is a part of the circuit on the left missing. Where it says V_rect, is a transformer and rectifier circuit, so v_rect should be a full wave rectified signal.

Hmm,
R2 = (5V - 3.3V)/5mA = 340 = 330Ω
But i think that you should start from R2 = 10K. You don't need to waste that much amount of current for no reason, because opamp input bias current is "very small" 200pA. So you could chose voltage divider current much larger then input bias current Idv >> 200pA.